Heretofore, cranes have been driven by DC motors. Using induction motors instead of the DC motors has been discussed, because induction motors are stout, easy to maintain, and inexpensive. When such an induction motor is employed, a slip-frequency control operation or vector control operation is performed to obtain characteristics comparable with the characteristics of a DC motor which are easy to control. In this case, the primary current and the slip frequency of the induction motor are controlled. Thus, it is possible to obtain characteristics comparable with the characteristics of a DC motor, including accuracy with which the torque is controlled and response.
When the vector control operation is performed, it is necessary to detect the slip frequency.
It has been customary to use a tachometer generator or pulse generator to detect the slip frequency. Today, investigations are conducted to simplify the structure and reduce the cost by arithmetically finding the slip frequency without using any of such detectors.
One method of driving an induction motor makes use of a inverter. This method is now described in detail by referring to FIG. 5. A main circuit is formed by a DC power supply 10, a smoothing capacitor 11, power transistors 12-17, and diodes 18-23. The voltage, current, and frequency applied to an induction motor 1 are controlled by turning on and off the power transistors 12-17. In this way, the velocity of the motor is controlled. A signal-processing circuit 24 that constitutes an inverter control circuit calculates the frequency and the voltage to be applied to the motor 1 in response to a velocity reference N.sub.REF. Then, the circuit 24 determines the pulse duration and the pulse repetition frequency to maintain the ratio of the voltage to the frequency constant. Where the motor is a three-phase induction motor, the signal-processing circuit 24 delivers a three-phase PWM (pulse width modulation) signal 25.
A base driver circuit 26 has a pulse transformer or photo-coupler for isolating the main circuit power supply from the control power supply. The driver circuit 26 further includes an overcurrent-detecting circuit for detecting overcurrents flowing into the power transistors. The base driver circuit 26 produces a base driver signal 27 for the power transistors 12-17 of the main circuit from the modulated signal 25. Indicated by numeral 28 is a protective circuit for the base driver circuit 26.
There exists an open-loop system in which a signal indicative of the speed of a motor is not fed back to the input, for controlling a V/F inverter. This system does not use a speed detector such as a tachometer generator, in order to curtail the cost.
When such an open-loop system for controlling a V/F inverter is employed to control the operation of a crane, when the crane is started, the velocity of the load does not increase because of a lag in the operation of the mechanical brake, but there arises the possibility that the load is dropped since the output frequency of the inverter increases. Also, when the crane is stopped, the load might slide down because the mechanical brake is not immediately actuated.
For these reasons, the common practice is to use a speed detector, such as a pulse generator, and an V/F inverter for controlling the slip frequency or the stator current vector of an induction motor.